A bit more detail: the assembly included a rod (of an precise length) that was inserted into a socket which consisted of a ring that was welded or braised to something else. The fastening technique left a small amount of slag or solder around the inner edge of the ring, and so the rod was chamfered on one end so that it would clear the slag and seat fully in the socket.
Problem was, the rod was inserted upside down, with that attractively chamfered end sticking out. And the neatly cut business end ended up bottoming on the slag, resulting in a part that was precisely built but off by 1.3 mm.
When they eventually tracked down the problem, they were able to use the mis-aligned jig as a model for designing the corrective mirror that they eventually installed.